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MTX Co-trimoxazole Interaction — RACP Adult Medicine MCQ

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EasyClinical PharmacologyMTX Co-trimoxazole InteractionRACP Adult Medicine

A 50-year-old woman with dermatomyositis on prednisolone and MTX develops oral ulcers and pancytopaenia. She started co-trimoxazole 5 days ago. What has occurred?

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Correct answer: A — MTX toxicity potentiated by co-trimoxazole

Co-trimoxazole + MTX: additive antifolate toxicity + reduced MTX renal clearance. Folinic acid rescue needed. Avoid combination.

Reference: AMH – 2025; eTG – 2025